Illustrated herein in embodiments are crosslinking processes, and more specifically, in situ crosslinking processes, for making polyester resin emulsions containing crosslinked polyester resins. More specifically, in embodiments is provided a process to produce crosslinked polyester emulsions wherein an unsaturated polyester resin is crosslinked in situ in a solvent flashing emulsification process utilizing free radical initiators. Tile emulsions can be used, for example, to produce toner compositions that exhibit excellent fusing performance, excellent relative humidity sensitivity and high temperature/high humidity charging performance, while still providing desirable gloss properties.
To achieve desired ultra low melt performance in various toner formulation applications, it is advantageous to utilize a blend of crystalline and amorphous polyester resins in the toner formulations. Crystalline resins alone in toners provide excellent low melt and high gloss performance, but tend to provide poor fusing latitude. Amorphous resins alone in toners provide excellent release performance, but their low melt performance is limited by blocking and document offset requirements. By mixing both crystalline and amorphous resins, it may be possible to achieve both ultra low minimum fix temperature and wide fusing latitude.
In one approach, amorphous resins comprised of linear or branched saturated polyester resins have been used in toner formulations that resulted in excellent gloss and fusing performance when used in combination with unsaturated crystalline polyester resins. However, this tends to result in poor relative humidity (RH) sensitivity and high temperature/high humidity (80° F./80-85 percent RH) charging performance.
In another approach, polyester based toners comprised of linear unsaturated resins such as for example propoxylated bisphenol A fumarate resin and unsaturated crystalline resin have been provided. These toners show excellent fusing performance as well as excellent RH sensitivity and high temperature/high humidity charging performance. However, the gloss is very high as compared with conventional toners, and document offset is poor. It is known that increasing resin glass transition temperature (Tg) can reduce gloss and improve document offset. This, however, results in larger resin emulsion sizes during resin emulsification, making it more difficult to produce the toners. In the case of styrene-acrylate emulsion aggregation toner processes, resin emulsions comprised of crosslinked resin particles such as, for example, generated from the emulsion polymerization of styrene, butyl acrylate, divinylbenzene and beta carboxy ethyl acrylate are utilized to reduce gloss. There has been, however, no known method to produce similar emulsions containing crosslinked polyester resins.
A known process for emulsifying polyester resins is by solvent flashing wherein the resin is dissolved in an organic solvent such as for example ethyl acetate at an elevated temperature but below the boiling point of said solvent such as for example 65° C. The resulting solution is mixed into water containing an anionic surfactant such as Taycapower BN2060 (Tayca Corp., Japan), mixed with a homogenizer and then heated to a further elevated temperature above the boiling point of said solvent such as for example 80° C. to flash off the solvent and then cooled to room temperature.
Further known solvent flashing emulsification processes for polyester resins utilize bases such as, for example, sodium hydroxide or ammonium hydroxide, as the stabilizer with reduced or substantially no surfactant. Such processes have the added advantage of reducing the need to remove the surfactants in toner washing processes such as to enable satisfactory toner charging and development performance.
Unfortunately, if one was to attempt emulsification of crosslinked polyester resins by the known solvent flashing methods, major difficulties would be encountered because the crosslinked polyester is substantially not soluble in most common solvents.
The processes of the disclosure, in embodiments, provide a means for the preparation of toner compositions containing crosslinked polyester resins wherein the crosslinking is carried out in situ in the solvent flashing process during resin emulsification, and thereby circumventing the difficulties encountered in emulsifying polyester resins which have already been crosslinked.